I had a fairly major knee injury bout 10 years ago (tore my anterior cruciate ligament) playing squash and its never really been stable since.
The good news is that I am still a gym junkie but I stay away from any activities that carry a risk of any sideways movement eg step classes or other free moving aerobics.
I LOVE the RPM (or spin) and pump is fine too (i take it very easy with lunges). So biking is great if you have a knee injury like this.
Guess it depends what your exercise passions are.....and you will get to know what is most kindest on your knees!
I want to get out into the world.......NOW!